portable generator grounding/bonding

[electrofelon]

We are hooking up a service with a 200A DPDT transfer switch for connection to a portable backup generator. My understanding is that since the neutral is not switched, this is not a SDS, and thus a four wire feeder is to be run to the generator and the N-G bond at the generator (if present) is to be removed. Questions (assuming the previous is correct):

1. What is the purpose of seperating the N-G bond on the genny? Consider a fault to the generator frame; with the N-G bond removed, the path back to the source is via the genny feeder EGC, then potentially a 'sub panel' feeder EGC to the service equipment, then all the way back on the neutrals to the genny. What is the health and safety concern that makes removing the bond a safer install despite the drastically longer fault path?

2. Another problem with removing the N-G bond: Without the cord connected to the generator, or when the HO uses the generator somewhere else, there is no fault path if the frame becomes energized. Right? Whats going on here?

3. What does "neutral floating" mean.

 

[benaround]

Electorfelon,

250.34(C) takes you to 250.20(D) , then in the NECH 2005 to exibit 250.6

and that answers your question. Having the system GC and the gen. GC tied

together at the transf. sw. provides grounding for the gen. GC. no need to

ground the neutral at the gen. Make sure there is an EGC from the sevice to

the transf. sw.

 

[dcspector]

Neutral "floating" generally is reffering to not bonding the neutral to a frame, cabinet, etc. ie. a neutral bar in a sub panel raised in the cabinet with a non conductive material. Best way I can explain it.

 

[dnem]

Questions (assuming the previous is correct):

Yes, the previous is correct

1. What is the purpose of seperating the N-G bond on the genny? Consider a fault to the generator frame; with the N-G bond removed, the path back to the source is via the genny feeder EGC, then potentially a 'sub panel' feeder EGC to the service equipment, then all the way back on the neutrals to the genny. What is the health and safety concern that makes removing the bond a safer install despite the drastically longer fault path?

Yes, the fault path is long but this set up allows it to function properly. . When there's more than one bonding point between the neutral and the grounding, you will have current flow on the equipment ground which is considered the less safe installation. . All separately derived systems can only have one single bonding point or they will have current flow on the ground on some sections in the system. . You would have the neutral and the equip ground in parallel if they're bonded together at more than one point.

If the transfer switch breaks and switches the neutral, the generator keeps the neutral-ground bond because the neutral and the equip ground are not in parallel. . There is no continuous path on the neutral between generator bonding point and the utility service disconnect bonding point. . The equip ground is the only continuous path between the two power supply points.

2. Another problem with removing the N-G bond: Without the cord connected to the generator, or when the HO uses the generator somewhere else, there is no fault path if the frame becomes energized. Right? Whats going on here?

The generator cord will still have an end cap with separate neutral and equip ground so will have to be plugged into a syatem that is also not separately derived.

It's the other way around that would concern me. . I can't see that a switched neutral transfer switch would be safe for a portable generator setup. . What if you plugged in a "subpanel" wired generator that had neutral and grounding separation ? . You would have a floating neutral under load.

3. What does "neutral floating" mean.

"Floating" on a portable generator means that the neutral is floating while the generator isn't plugged in. . The neutral isn't connected to anything other than the windings on the generator until you plug it in. . The cord not only connects it to the load but also to the main bonding point in the main disconnect.

If you plugged in a "subpanel" wired generator to a separately derived switched neutral transfer switch, you would have a "floating" neutral under load , which would be very dangerous !

David

 

[George Stolz]

Questions (assuming the previous is correct):

It is correct.

1. What is the purpose of seperating the N-G bond on the genny?

If you didn't, it would provide a parallel neutral path for equipment on the load side of the normal service. It would introduce "objectionable current" to the EGCs, and violate 250.6, among others.

The fault current path should be taken into consideration. Some inspectors on the forum would use 250.4 to fail an installation where the fault path isn't as good as it should be. This can be compensated for with larger conductors.

2. ...when the HO uses the generator somewhere else...

If that is a potential application, then either the homeowner needs training on the setup or you need to install a 3-pole switch and disconnect the neutral with the transfer switch. The last option is probably better.

 

[ramdiesel3500]

I have a 3-pole manual transfer switch installed at my own house for this very reason! I switch the neutral connection which makes this a separately derived system. Now I can use my portable generator for other applications without the need to disconnect/reconnect the bonding jumper inside the alternator housing. I believe rule 250.34(C) requires that I connect a grounding electrode conductor to my generator frame whenever I connect it to the house. I have not been doing this because, physically, the generator frame is connected to the grounding electrode conductor via the grounding conductor. Should I get myself another ground rod clamp and a piece of #6 awg bare copper to make a connection from the generator frame to the grounding electrode? This way, during a power outage, I would need to plug my transfer switch into the generator (L14-20 plug with 4-wire #12 cord) AND connect the #6 AWG Bare copper grounding electrode conductor to the generator frame.
What would you do?

 

[tom baker]

I would not transfer the neutral, have a system bonding jumper in the portable generator, and not worry about the objectionable current on the GC.
The CMP understands this issue, and there feeling is its better to have the SBJ than not.

 

[electrofelon]

Thanks for the replies/explainations everyone. I see the reasoning for not making the N-G bond at the genny as analagous to the reason for not making a N-G bond (at most locations) in a premis' wiring system.

The floating neutral is still a bit fuzzy however: my generator says neutral floating on it, so does this mean the frame could become energized and i would not know until it bit me (assuming it is not connected to a structure which will make the neutral-ground bond at the structure's service equipment thus allowing the OCPD to trip)? For that matter, it wouldn't just be the frame of the generator, non current carrying parts of utilization equipment could become energized with no OCPD activation - I dont see how that could be, I must not be understanding something. Again, here I am talking about powering a washing machine in the middle of a large field with a generator - no premis interconnect. As a side note, would the NEC cover this situation?

 

[don_resqcapt19]

2. Another problem with removing the N-G bond: Without the cord connected to the generator, or when the HO uses the generator somewhere else, there is no fault path if the frame becomes energized. Right? Whats going on here?

Many of the small portable generators do not have this bond installed at the factory. There is no hazard and if one of the two floating conductors becomes grounded, at that point it becomes a grounded system.
Don

 

[iwire]

I would not transfer the neutral, have a system bonding jumper in the portable generator, and not worry about the objectionable current on the GC.
The CMP understands this issue, and there feeling is its better to have the SBJ than not.

Tom that is a direct NEC violation.

If the CMP feels that way they better change the rules.

 

[Pierre C Belarge]

Generators that have a floating neutral, does not mean to me that the system is ungrounded. What that says to me is the manufacturer did not make the 'neutral to ground' connection at the generator. The EGC installed with the conductors from the building will provide the ground pathon that side of the generator, and the grounding at the generator will provide the generator system with a grounded neutral.

Of course there may be some situations where the generator is used with an 'ungrounded system', but that is not the majority of installations in this country.

 

[electrofelon]

Many of the small portable generators do not have this bond installed at the factory. There is no hazard....

But isnt this just like a premis where the main bonding jumper is missing - the circuit will not be deactivated? Generators are used extensively to run tools directly, say at a job site with no power, and if the tools are not double insulated it sounds like a shock waiting to happen. I admit this is beyond the scope of the NEC, but it seems unsafe to not have a N-G bond at the generator for stand alone use.

 

[LarryFine]

But isnt this just like a premis where the main bonding jumper is missing - the circuit will not be deactivated? Generators are used extensively to run tools directly, say at a job site with no power, and if the tools are not double insulated it sounds like a shock waiting to happen. I admit this is beyond the scope of the NEC, but it seems unsafe to not have a N-G bond at the generator for stand alone use.

It's not really worse than a non-grounded Delta.

In fact, a person is less likely to receive a shock when the system is ungrounded, becase there's no voltage between any of the generator conductors and earth.

Now, hold a defective tool and touch the generator . . .

 

[suemarkp]

The difference is that the power company transformer is grounded, as is one of most of their other distribution wires. So power company feeds almost always have a voltage relative to earth.

A portable generator that is not earthed in any way does not generally pose a shock hazard in the way you're used to thinking about it. It isn't connected to the earth, so standing in a puddle and touching one wire isn't going to hurt you.

 

[iwire]

Now, hold a defective tool and touch the generator . . .

If the generator has a floating neutral nothing should happen.

 

[LarryFine]

If the generator has a floating neutral nothing should happen.

Oh, right.

Okay, now hold a defective tool while touching a defective generator . . . :wink:

 

[don_resqcapt19]

Pierre,

Generators that have a floating neutral, does not mean to me that the system is ungrounded. What that says to me is the manufacturer did not make the 'neutral to ground' connection at the generator.

The ones that I have seen are portable generators designed for cord connected equipment...it is an ungrounded system.
Don

 

[iwire]

Oh, right.

Okay, now hold a defective tool while touching a defective generator . . . :wink:

50 / 50 shot that both have the same phase fault.

 

[iwire]

Pierre,

The ones that I have seen are portable generators designed for cord connected equipment...it is an ungrounded system.
Don

I agree.

I have a 5000 watt Coleman generator designed for portable use, it is labeled "floating neutral" it is sold for the average consumer.

When I use it I am using an ungrounded system.

 

[LarryFine]

50 / 50 shot that both have the same phase fault.

All right, already!

Hold a defective tool while touching a defective generator while they have the same-phase fault . . .

Happy? Geez! :wink: